Matrix is an open standard for decentralized persistent communication over IP.
It can be used for
decentralised group chat,
WebRTC signaling,
IoT-data transfer,
and generally to link together fragmented silos of communication.

Riot (formerly Vector) is an opensource interoperable app running on Web, iOS and Android. It is built around chat rooms, both public and private and provides messaging, filesharing, voice and video conferencing (beta on Android and web), integrations to tools like GitHub, Jira, Jenkins and much more soon. Vector is built on the Matrix.org open standard which provides bridges to other systems such as Slack, IRC and a lot more soon. One can use the hosted service or run their own.

Jitsi (previously SIP Communicator) is an audio/video and chat communicator with full end-to-end encryption that supports protocols such as SIP, XMPP/Jabber, ICQ/AIM, Windows Live, Yahoo!, GTalk/Hangouts extensions, as well as OTR, ZRTP, etc.
It can handle every firewall, and has many other useful features.

Jitsi is completely open source / free software, and is freely available under the terms of the GNU Lesser General Public License.

An open-source decentralized and encrypted messaging app even without internet connection via Wi-Fi and Bluetooth.
Messages are stored securely on your device, not in the cloud. Connect directly with nearby contacts - no Internet access is required.

Using Signal, you can communicate instantly while avoiding SMS fees, create groups so that you can chat in real time with all your friends at once, and share media or attachments all with complete privacy. The server never has access to any of your communication and never stores any of your data.

Speak Freely - Make crystal-clear phone calls to people who live across town, or across the ocean, with no long-distance charges.

Be Yourself - Signal uses your existing phone number and address book. There are no separate logins, usernames, passwords, or PINs to manage or lose.

Stay Secure - We cannot hear your conversations or see your messages, and no one else can either. No exceptions. You can even tap and hold on a contact's name to see advanced identity verification options. Everything in Signal is always end-to-end encrypted and painstakingly engineered in order to keep your communication safe.

Go Fast - Signal is optimized for speed without compromise. Our state-of-the-art Axolotl protocol is unmatched in its performance, strength, and reliability.

Remain Connected - Push notifications let you know when new messages have arrived, and they'll be waiting for you even if your battery dies or you temporarily lose service.

Get Organized - Unique archive functionality makes it easy to keep track of the conversations that matter to you right now.

Pay Nothing - Signal is supported by a team of dedicated developers, community donations, and grants. There are no advertisements, and it doesn't cost anything to use.